I think there are two requirements.
One is a map of the Island showing the parishes, roads, etc.
The map that is presently available was (I think) provided by the FHS and really does not tell me a great deal more than that Rushen is in the south and Bride in the North (that's a bit of an overstatement, but you get the idea). As to scale, I would think something that would take up about 1/3 to 1/2 an A4.

The second would be individual Parish maps, which I had not thought about. Probaly more of interest to those tracing properties. Woods may be the answer here as the one example I've seen would appear to lend itself to easy identification with modern landmarks.

I'd love to see the one covering Baldwin, I'm looking for a farm at Ulican.